Many electrical devices, such as cell phones, personal digital assistants (PDAs), laptops, and the like, are powered by relatively low-voltage, direct-current (dc) power sources. Since power is typically delivered through a wall outlet as high-voltage, alternating-current (ac) power, a device generally referred to as a switching-power converter is typically used to transform the high-voltage ac power to low-voltage dc power. These converters generally use a controller to switch a power switch between an ON state and an OFF state to control the amount of power transmitted to the output of the converter and delivered to a load. Specifically, when the power switch is in an ON state current may be able to conduct through power switch, and when the power switch is in an OFF state current may not be able to substantially conduct.
In some applications, switching-power converters may include an energy transfer element to transfer energy between an input side and output side of the power converter. More specifically, an energy transfer element can be used to provide galvanic isolation, which prevents dc current from flowing between the input and the output sides of the power converter. Common examples of energy transfer elements include transformers and coupled inductors, where electrical energy is transferred through magnetic coupling between windings.
One type of power converter, referred to as a primary-side regulated power converter, obtains feedback information using a bias winding that is electrically coupled to the input side of the power converter such that the bias winding is also magnetically coupled to the output winding of the energy transfer element. This allows the bias winding to produce a voltage representative of the output voltage of the power converter. In this manner, the switching-power converter can acquire a feedback signal representative of the output voltage without directly sensing the output voltage at the output of the converter. During operation, the power converter may regulate an output quantity by using one or more various control techniques that may adjust the frequency and/or duration of the switching events in response to the feedback from the bias winding. By adjusting the frequency and duration of the switching events, the converter can control the amount of energy transmitted from an input of the power converter to the output. A switching event may be defined as when the power switch transitions between an ON state (switch closed) and on OFF state (switch open).
During a light or no load condition the power converter must maintain the regulated output voltage with minimal power. As such, the controller limits the energy transfer to the output by adjust the switching of the power switch by increasing the time between switching events to provide the regulated output voltage with only the minimal power required. However in this circumstance, the controller used for primary side control may not receive feedback from the output in the absence of switching and therefore cannot sense the output voltage during long intervals between switching events. In the event, when an increased load is suddenly applied to the output of the power converter during one of the long intervals between switching events, the output voltage may drop below the specified regulated output voltage before the controller has the opportunity to receive feedback information and can respond to the increased load condition.
In one known embodiment, increasing the bulk capacitance connected across output voltage terminals to provide energy required by a sudden load increase during the dormant switching interval may allow for proper regulation with larger intervals in between switching events. However, the increasing the bulk capacitance affects size, cost, and efficiency of the power converter design, which are undesirable.